I have'nt used a fisheye adapter but did use a similar wide angle one. It was ok - it just made everything a bit less sharp, particularly in the corners. That probably won't matter quite so much with a fisheye effect and, of course, it is much cheaper than a dedicated fisheye lens. It would probably be worth trying out to see if you like the effect and then consider getting a true fisheye. I have one (a Nikkor 10.5mm converted to work on a full frame sensor) and I do use it a reasonably amount as the effect can work really well with some subjects. I just wouldn't use it for everything.